There have been extensive efforts directed towards the preparation of various polymer-clay composite materials since the discovery of exfoliated nylon/clay nanocomposites by Usuki et al. (J. Mater. Res. 1993, 8, 1174). Such materials are expected to have new and improved mechanical, thermal, and barrier properties compared to those of the polymers alone.
Incorporation of a few percent of clay has been found to greatly increase a polymer's modulus, strength, gas barrier properties, and heat distortion temperature. The presence of clay is also reported to impart fire retardant properties. Without wishing to be bound by theory, the improvement in thermal stability is believed to be attributed to tortuous diffusion of the volatile thermal and therm-oxidative decomposition products in the presence of dispersed silicate layers. The slower diffusion of decomposed volatiles along with structural reinforcement provided by the char formed with collapsing silicate layers during combustion contributes to decreased flammability of exfoliated nanocomposite as demonstrated by cone calorimetry. See for example, J. Zhu and C. A. Wilkie Polym. Int. 2000, 49, 1158; and J. W. Gilman Appl. Clay Sci. 1999, 15, 31.
The most common morphology for miscible polymer-clay dispersions is known as intercalation. In this case, the host polymer penetrates the space between the clay platelets, but separating them only slightly and maintaining the parallel, regular structure of the platelets. Intercalated polymer-clay nanocomposites are often observed to have measurable improvements in physical properties, but typically less so than if the corresponding nanocomposites were in the morphology known as exfoliation. Although exfoliation is much more desirable, it is less common and more difficult to obtain. In this morphology, the clay platelets are thoroughly separated from each other by the host polymer, so that their original crystallographic register is lost. Particularly for nonpolar polymer hosts, the fully exfoliated polymer-clay nanocomposites are notoriously difficult to obtain.
Non-polar polymers, including polystyrene and polyethylene, represent a group of commercially important thermoplastics. Polystyrene and linear low density polyethylene (LLDPE) homopolymers form intercalate morphologies when melt blended with organically modified clays known as organoclays. See, for example, R. A. Vaia and E. P. Giannelis Macromolecules 1997, 30, 8000, and also K. H. Wang et al. Polymer, 2001, 42, 9819.
Although exfoliation of clay in these non-polar polymers is more desirable, achieving this state of morphology is particularly challenging because the polymers are not strongly attracted to the clay surfaces. One approach to achieve this goal is in-situ polymerization of non-polar monomers in the presence of organoclay bearing either polymerizable functional groups or initiators. See, for example, X. Fu and S. Qutubuddin Polymer 2001, 42, 807; J. Zhu et al, Chem. Mater. 2001, 13, 3774; and M. W. Weimer et al. J. Am. Chem. Soc. 1999, 121, 1615.
Another method to achieve exfoliation of such non-polar polymers is to incorporate polar units in the backbone of non-polar guest polymers and subsequently melt blend them with a host organoclay. See, for example, N. Hasegawa et al. J. Appl. Polym. Sci. 1999, 74, 3359; and C. I. Park et al. Polymer 2001, 42, 7465. The latter strategy is industrially more feasible, but increasing the fraction of polar units in non-polar polymers may result in undesirable morphological changes, e.g., phase separation. Thus, complete exfoliation of organoclay in non-polar homopolymer has not been commercially or economically practicable on a comparable scale.
U.S. Pat. No. 7,166,657 (Rao et al.) describes the use of highly branched polar polymers to splay various clays to form nanocomposites for various uses.
U.S. Pat. No. 6,686,407 (Robello, et al.) describes the use of star-branched polystyrene to achieve exfoliation of organoclays. A theoretical treatment of this approach is given by Singh and Balazs (Polym. Int. 2000, 49, 469). Exfoliated nanocomposites were achieved, but such star-branched polymers are laborious and expensive to synthesize and a more practical technique is highly desirable.